An image sensor is a semiconductor device for converting optical images into electric signals, and is classified into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor includes a photodiode and a MOS transistor in each unit pixel, and sequentially detects the electric signals of each unit pixel in a switching mode to realize images.
The CMOS image sensor typically includes a metal interconnection layer, a passivation layer, a color filter array, a planarization layer, and a micro-lens formed on a semiconductor substrate formed with a photodiode. In such an image sensor, as a device reduces in size and becomes highly integrated, the size of a unit pixel is proportionally reduced. In addition, the size of a photodiode, which is a photo response region, may be relatively reduced.
The image sensor has a limitation where reducing a pixel size is inversely-proportional to saturation electron capacity.